The mammalian cerebral cortex is a layered structure. I am interested in the molecules that regulate development of cortical layers. The first specific aim is to identify genes that are critical in the formation of proper axonal projection patterns for neurons within a specific cortical layer, layer 5. Layer 5 neurons initially extend axonal branches to many subcortical areas. Later the exuberant branches are selectively pruned so that a proper area-specific projection pattern is formed. The homeodomain transcription factor, Otx1, is required for the pruning of exuberant axonal branches. However nothing is known about the target genes of Otx l or about the mechanisms of axonal pruning. I propose to identify genes that act downstream of Otx1 to regulate axonal refinement using cDNA microarray technology. The second aim is to isolate and study genes which regulate the initial laminar fate determination or early stages of laminar differentiation. I have finished an in situ hybridization screening and identified twelve novel as well as known genes that demonstrate layer-specific expression patterns. Experiments are proposed to study their potential functions in laminar fate determination and/or early stages of layer differentiation.